1. Field of the Invention
The present invention relates generally to a wireless communication system, and more particularly, to an apparatus and a method for calibrating a sounding signal of a mobile station in a wireless communication system supporting a cooperative Multiple Input Multiple Output (MIMO) scheme.
2. Description of the Related Art
Constant efforts have been made to mitigate inter-cell interference in cellular wireless communication systems. Recently, a scheme for reducing inter-cell interference more actively through cooperation of base stations has been under development. Particularly, cooperative Multiple Input Multiple Output (MIMO) schemes, which extend various MIMO schemes applied to a single cell to a plurality of base stations, are suggested. To use the cooperative MIMO scheme, the base stations require channel information of each mobile station. In general, an amount of feedback corresponding to the required channel information is proportional to a performance of the MIMO scheme. Accordingly, to maximize the performance of the cooperative MIMO scheme, a considerable amount of Channel State Information (CSI) feedback is required.
In a Time Division Duplex (TDD) system, base stations can acquire channel information using a sounding training signal without a large amount of data feedback. When a cooperative MIMO scheme is applied, the base stations can simultaneously acquire the channel information using the sounding training signal, which is transmitted only once by the mobile station. Thus, overhead in using the sounding training signal can be greatly reduced. In order for the base stations to use the sounding training signal, calibration for synchronizing transmit and receive Radio Frequency (RF) chains between the base stations and the mobile station must be performed.
The calibration is used to obtain reciprocity between an uplink channel and a downlink channel, and to compensate for a magnitude and phase shift of signals generated in the transmit RF chain and the receive RF chain of a transmitter and a receiver. Hence, when sounding is applied, the calibration must be performed. Various conventional schemes are suggested for performing sounding. However, the conventional calibration schemes target a transmitter and receiver in a single cell, and do not consider sounding performed according to a cooperative MIMO scheme. Even when the calibration is conducted on a per-cell basis according to the conventional calibration schemes, calibration complex constants of base stations participating in a cooperative MIMO differ, and thus, cooperative MIMO performance degrades.